Powertrain, generic (extended)

P21BD Cold Start Cylinder 8 Injection Timing

Check engine lightOn

EmissionsRelevant

Drive cycleRequired

Limp modeNo

P21BD covers cylinder 8, the last cylinder of a V8. GM's 2025 diesel tables show how close an existing monitor comes to this code: during cold start the module compares requested and scheduled start of injection, with every cylinder from A to H included in the enable checks, and a deviation beyond ±4 in 88 of 176 samples sets the lamp in the first trip. Yet GM stores only the system code P05EC, which does not say which injector misbehaved. A per-cylinder code like P21BD would point straight at cylinder 8; no manufacturer documents its use publicly. GM's petrol V8 with direct injection likewise use a special dual-pulse cold-start mode. Related: P021B (warm), P2B0F (cold-start pulse), P0208, P0308.

Likely causes

Injector of cylinder 8 with slow or varying response

Connector or wiring fault at injector 8

Cold-start calibration in the engine module needing an update

Symptoms

Check engine light after cold starts
Rough running or smoke in the first minute after a cold start
No symptoms at operating temperature

Repair estimate

DifficultyHardWorkshop job

Parts and labour€120–1,600

Time1–5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

fuel injectorinjector wiringengine control module

References: law.cornell.edu, gsi.ext.gm.com, static.nhtsa.gov.